How I lost my best friend

I was having the time of my life on vacation when the guy I was seeing and I decided that it was over.
A few weeks earlier, I had introduced my best friend to him while on a group date. Her having an amazing boyfriend, I didn't think anything of it. Until she tells me that they have been hanging out while I was gone.
I wouldn't say I was jealous, just more annoyed. As my best friend, I thought she would have respected me enough to not befriend my ex, whom has clearly hurt me.
To make matters worse, when I got home she pulls me aside and tells me that he kissed her, and that she wasn't going to tell her boyfriend, who was in South Africa working with the orphans.
After a 4 hour heart-to-heart with her, she decides to breakup with her amazing boyfriend, in order to attempt to sleep with me ex. At this point, I am furious, however I thought she had enough decency to wait until her boyfriend got back from his mission trip. Nope! She breaks up with him over Facebook, and then sleep with my ex the next day.